The present invention relates to a circuit for driving a mechanical device at its resonance frequency. More particularly, the present invention relates to a circuit for automatically driving a device at its resonance frequency.
Various types of audible indicators that employ a piezoelectric or electro-mechanical transducer to generate a relatively piercing and noticeable audible tone when energized with power have been used for many applications. Such indicators are commonly used in numerous small and large appliances and alarm systems, and for other applications in which the generation of an audible signal is required. For example, for safety reasons, many heavy duty machineries such as forklifts and bulldozers include a backup alarm system that will generate a loud, and sometimes offensive, warning signal during their operation in the reverse driving mode so as to warn passersby of their movement.
During their operation, these alarm systems are preferably operated at or near the resonance frequency of the vibrating element even though such alarm systems may be operated at other frequencies. By operating at or near the resonance frequency, the most efficient use of available electrical energy to produce the greatest audible output is achieved. As a result, manufacturers often will test their alarm systems and, if necessary, adjust or tweak such alarm systems to produce the maximum audible output. Although this manufacturing step is implemented as a quality control step to ensure that each alarm system leaving the factory will operate at its maximum efficiency, the resonance frequency of each alarm system may later vary due to such factors as aging, and varying temperature and humidity. In light of such previously-stated problem, various alarm systems have been proposed so as to operate at or near a resonance frequency at any time during their usage. These proposed alarm systems are generally complicated and costly.
Accordingly, it is desirable to eliminate the above-mentioned labor-intensive manufacturing step of testing each alarm system to ensure that each alarm system leaving the factory will operate at its maximum efficiency, especially when the resonance frequency later may vary due to uncontrollable factors. By reducing such step in their manufacturing process, makers of alarm systems can effectively reduce the costs associated with the production of these alarm systems. In addition, it is also desirable to provide a simplified circuit capable of automatically driving the vibrating element of these alarm systems at or substantially near a resonance frequency so that minimal electrical energy is used to produce the greatest audible output.
The above-mentioned labor-intensive testing step is further associated with the production of [1] wireless RF xe2x80x9ckey fobsxe2x80x9d for car security alarm systems and [2] remote control garage door openers. In order to transmit signals, the wireless RF key fobs and remote control garage door openers include a signal transmitting device such as an antenna. Although very little power is required to drive the antenna, it is still desirable to extend the life of the battery providing such power. Thus, prior to their shipment from the manufacturers to the wholesalers or retailers these wireless RF key fobs and remote control garage door openers are also tweeked or adjusted for maximum power efficiency. Similar to the alarm systems, maximum power efficiency of the wireless RF key fobs and remote control garage door openers is achieved when the antenna is driven at a resonance frequency.
Accordingly, it is also desirable to eliminate the above-mentioned labor-intensive manufacturing step of testing each wireless RF key fob or remote control garage door opener by providing a circuit capable of automatically driving the antenna at or substantially near a resonance frequency so that minimal electrical energy is used to transmit signals.
Generally, the present invention is directed to a circuit for automatically driving a mechanical device at its resonance frequency. To do so, the circuit detects non-resonance driving conditions of the mechanical device being coupled to and driven by such circuit. Based on such detection, the circuit generates a signal to drive the device at its resonance frequency.
More specifically, according to one aspect of the present invention, an acoustic transducer system is provided. The acoustic transducer system comprises [1] a power supply, [2] an acoustic transducer having a first electrical terminal coupled to the power supply and a second electrical terminal coupled to a reference ground, and [3] a phase-locked loop circuit detecting a phase difference between first and second signals at the first and second electrical terminals, respectively, and generating an output signal based on the detected phase difference to drive the acoustic transducer via a feedback connection forming a closed loop from the phase-locked loop circuit back to the second electrical terminal. The output signal generated by phase-locked loop circuit drives the acoustic transducer at a resonance frequency when the detected phase difference is negligible.
According to another aspect of the invention, a circuit automatically drives an antenna coupled to the circuit at a resonance frequency when a power supply is provided. This circuit comprises [1] a major feedback circuit providing an output signal, [2] a power amplifier driving the antenna in response to the output signal of the major feedback circuit, wherein the major feedback circuit detects a frequency difference between its output signal and a reference signal being provided to the major feedback circuit, and [3] a minor feedback circuit, coupled to the antenna and the major feedback circuit, detecting a phase difference between voltage and current signals provided by the power amplifier to drive the antenna, wherein the major feedback circuit generates the output signal based on the detected frequency and phase differences, and further wherein the power amplifier drives the antenna at the resonance frequency when the detected phase difference is negligible.
These and other features and advantages of the present invention will be apparent from the drawings as fully explained in the Detailed Description of the Preferred Embodiments.